This invention relates to an orthopedic cast material which, when wrapped about a body member and cured by exposure to visible light, forms a rigid, high strength immobilizing structure and which comprises an air, light and X-ray permeable fabric impregnated with a formulation comprising: (a) acrylate terminated polyurethane oligomers, (b) functional acrylate diluent monomers as optional ingredients and (c) a photoinitiator which is activated by visible light to initiate the polymerization reaction, whereby the casts produced therefrom show no tendency towards discoloration in ambient light or sunlight. This invention also relates to the formulation used for preparing the cast material, and the method of forming the cast as well as the cast itself.
Previously, a leg, arm or finger was immobilized by applying a Plaster-of-Paris bandage which subsequently dried to form an immobilizing orthopedic cast. Such bandages suffer from the disadvantage that the resultant casts are heavy, uncomfortable to wear, insufficiently permeable to air and, once set, they rapidly lose their strength when brought into contact with water. Furthermore, such casts are generally impermeable to X-rays so that if the body member is to be examined by X-rays the cast must first be removed from the body member. In such cases, when the cast has been removed it may be found that the body member has not healed sufficiently and it may then be necessary to reapply a bandage to the body member and reform a cast. This is obviously inconvenient.
Many attempts have been made to provide bandaging materials which are free from the above disadvantages. It has, for example, been attempted to impregnate bandaging material with polymer solutions which harden under exposure to ultraviolet light. The ultraviolet lamps required for this purpose are difficult to handle and, moreover, the ultraviolet light only reaches the upper layers of the bandage so that the deeper layers harden only after a considerable time, if at all. The use of ultraviolet radiation suffers from the further disadvantage that it is known to be harmful to the human eye and skin (e.g. burns) and when using ultraviolet radiation it may be necessary to take considerable care to shield the patient and the operator from the radiation during formation of the cast.
In order to overcome the above-mentioned disadvantages, it has been proposed to use orthopedic casting bandages produced from polyurethane prepolymers and which are hardened, or cured, by exposure to visible light radiation. To circumvent the need for ultraviolet light curing, a number of moisture-cured polymer cast systems based on isocyanate-terminated urethane prepolymer compositions have been marketed. The polyurethane casting bandages suffer from some of the same disadvantages as the Plaster-of-Paris bandages. The curing of these cast bandages is initiated by immersion into water and afford lightweight, X-ray transparent, porous (breathable) polymer casts which cure sufficiently within fifteen to thirty minutes to a weight-bearing strength. While superior to Plaster-of-Paris in ease of application to body members, rapid setting characteristics, and permeability to X-rays, the polyurethane based bandages suffer from some disadvantages. For example, the polymerizable polyurethane prepolymer formulation contains a polymerization catalyst which is, in part, responsible for the reduced thermal stability of the casting bandages during prolonged storage, where the bandage may undergo premature polymerization and hardening prior to removal from the package.
Premature polymerization of the isocyanate-terminated urethane prepolymer can occur also if traces of moisture enter the package during storage. Accordingly, it is absolutely necessary that such prepolymer be prepared, coated onto bandage strips, and packaged in sealed containers under strictly anhydrous, or nearly so, conditions if one is to preclude hardening of the casting tape during storage and prior to actual use. Furthermore, it has been found that the cured urethane prepolymer based orthopedic casts are sensitive to discoloration (yellowing) on exposure to ambient visible light and, in particular, to ultraviolet radiation (sunlight).
By using visible light to effect the cure of the organic material to produce the immobilizing cast, it is unnecessary to protect the patient and operator from the source of light as would be the case wherein the organic material is cured by exposure to ultraviolet radiation. Consequently, the use of visible light is more convenient, particularly for the operator. Furthermore, the orthopedic cast produced from the impregnated fabric of the invention is lightweight and thus is convenient to wear and is permeable to X-rays. Visible light sources are, of course, relatively cheap and are readily available as contrasted with ultraviolet radiation sources.
The use of a visible light source to effect photocuring of ethylenically unsaturated resin compositions is disclosed in U.S. Pat. Nos. 3,874,376; 4,071,424 and 4,235,686. U.S. Pat. No. 3,874,376 relates to visible light photocurable resin impregnated fabrics for use in preparing orthopedic immobilization devices. A method is disclosed for producing rigid orthopedic casts by means of exposure to visible light (400-750 nm wavelength range) of fabrics of various types which are impregnated with compositions consisting of ethylenically-unsaturated resins and monomers, a photosensitizer activatable by visible light and certain photopolymerization accelerators. Although this patent broadly discloses photopolymerizable formulations and refers to a composition containing at least one photosensitizer activatable by visible light in the 400-750 nm wavelength range, actual photosensitizers disclosed are the diketone type, in particular, benzil, camphorquinone, .alpha.-naphthil and p-tolil. Other specific photosensitizers disclosed are fluorenone, uranyl salts of various types, combinations of manganese carbonyl and organic halides and a number of photosensitive dyes which absorb actinic radiation in the visible light wavelength range. The preferred photosensitizers of U.S. Pat. No. 3,874,376 are fluorenone or the .alpha.-diketones. However, the benzil, camphorquinone and fluorenone photoinitiators tend to impart an unaesthetic yellow coloration to the cast when used at concentrations required to give rapid visible light cures. U.S. Pat. Nos. 4,071,424 and 4,235,686 merely relate to different aspects of the invention disclosed in U.S. Pat. No. 3,874,376.
U.S. Pat. No. 3,613,675 discloses ultraviolet light photocurable resin impregnated bandages for orthopedic cast applications. The resins used are blends of ethylenically or acetylenically-unsaturated monomers and polymers with various polythiols, catalyzed with photoinitiators such as the benzophenones, acetophenone and methyl ethyl ketone. Curing is effected by means of exposure to sunlamps, sunlight or radiation from xenon lamps, thereby requiring protection of the skin of the patient from burn damage. Furthermore, the use of the photoinitiators of U.S. Pat. No. 3,613,675 tends to result in nonaesthetic, discolored polymer casts.
Ultraviolet curable orthopedic cast materials, impregnated with a photosensitizer and photocurable (meth)acrylate terminated urethane prepolymers containing at least two reactive carbon-carbon double bonds, are also disclosed in British Pat. No. 1,512,553. The compositions of the latter patent are cured rapidly by photoinitiated polymerization and crosslinking of the two or more (meth)acrylate groups and do not require use of volatile and odoriferous crosslinking comonomers, such as disclosed in U.S. Pat. No. 3,421,501 and 3,881,473. However, the major disadvantage of this polymer cast system is the use of ultraviolet light and the hazards associated therewith.
In view of the fact that the catalysts used in accordance with the prior art (such as those disclosed in U.S. Pat. No. 3,874,376) result in non-aesthetic polymer casts, it is indeed surprising that the catalysts utilized in accordance with the present invention bring about a cast which is essentially white in color and which does not yellow when exposed to ambient light or to sunlight. Furthermore, the method of the closest prior art (as exemplified by U.S. Pat. No. 3,874,376) requires the use of a reducing agent which is normally an amine, as well as a photosensitive catalyst. In accordance with the present invention, on the other hand, such reducing agent is not essential. In fact, Applicant has found that when amines are added, the depth of cure of the cast may not be as good and the surface of the cast may tend to be somewhat tacky. In accordance with one aspect of the present invention, Applicant has found that the addition of certain polyfunctional mercaptans, which participate in the reaction, result in dry surface cures.